text: Those carbenes that have been isolated to date tend to be colorless solids with low melting points. These carbenes tend to sublime at low temperatures under high vacuum.
  One of the more useful physical properties is the diagnostic chemical shift of the carbenic carbon atom in the C-NMR spectrum. Typically this peak is in the range between 200 and 300 ppm, where few other peaks appear in the C-NMR spectrum. An example is shown on the left for a cyclic diaminocarbene which has a carbenic peak at 238 ppm.
  Upon coordination to metal centers, the C carbene resonance usually shifts highfield, depending on the Lewis acidity of the complex fragment. Based on this observation, Huynh et al. developed a new methodology to determine ligand donor strengths by C NMR analysis of trans-palladium(II)-carbene complexes. The use of a C-labeled N-heterocyclic carbene ligand also allows for the study of mixed carbene-phosphine complexes, which undergo trans-cis-isomerization due to the trans effect.